Various organizations to include the various departments of transportation and the like, for example, throughout the U.S.A., Canada, and so forth, perform numerous asphalt extraction tests yearly for quality control and performance evaluation. The most critical step in the known test procedures is the quantitative extraction of bitumen from the asphalt paving mixtures. Other determinations useful in evaluating these mixtures include the particle size distribution of the aggregate after extraction and the hardness of the extracted bitumen after solvent removal (penetration test)
The procedure used currently by the Michigan Department of Transportation (MDOT), based on ASTM D 2172-81, involves soaking the paving mixture in a solvent at ambient temperatures, followed by separation of the solvent-bitumen mixture from the aggregate by a complex procedure employing a centrifuge and repeated solvent washings The solvents that have been found to be effective and are extensively used are trichloroethylene (TCE), 1,1,1-trichloroethane (111-TCE), and methylene chloride (MeCl2). Unfortunately, because of their chlorine content these solvents present a serious problem for hazardous waste disposal. In order to minimize this problem, the MDOT eliminated TCE from use for their field extractions, although it is still being used in their Lansing laboratory. A commercial solvent, BIOACT-DG-1, which is primarily limonene, a terpine hydrocarbon, has been substituted for TCE in the field testing because it does not have the hazardous waste disposal problem as does TCE, nor does it appear to present a significant health hazard to exposed personnel, even though some workers have experienced some irritation due to its odor. In addition, BIOACT-DG-1 can be used with essentially the same equipment as is used with TCE. However, BIOACT-DG-1 presents several disadvantages when compared with the commonly used solvents for such extractions, among which include the following: (1) It is significantly more expensive. (2) It extracts the bitumen more slowly. (3) Its high boiling point does not allow for its separation from the extract so that a penetration test may be performed (4) It apparently undergoes a phase separation when stored at low temperatures such as in the winter, giving a solvent with inconsistent properties.
The Vermont Agency of Transportation has used xylene in place of TCE, and although it performs satisfactorily as a solvent, it is reported to present a serious health hazard to personnel. Preliminary tests were conducted to determine the suitability of substituting BIOACT-DG-1 for xylene, which, again, indicated that BIOACT-DG-1 could function as an extractant of the bitumen, but it took twice as long to do this as did xylene and was significantly more expensive It should be noted that solvents with higher boiling points than TCE, such as xylene or toluene, would be very difficult to separate from the extracted bitumen without excessive heating, thus limiting penetration testing and so forth.
The Pennsylvania Department of Transportation (PDT) employs a method for the quantitative extraction of bitumen from bituminous paving mixtures that is a modification of AASHTO T-164, a method similar to ASTM D 2172-81. Extraction of the paving mixture in the PDT method is conducted with 111-TCE or with toluene.
Alternate methods for bitumen extraction involving solvent reflux or vacuum filtration also have been developed for use with the chlorinated solvents. However, these do not avoid or overcome the mentioned problems encountered by use of these solvents.
An apparatus useful in such tests as the ASTM D 2172-81 and AASHTO T-164 is the Gilson Vacuum Extractor Model HM-8 (Gilson Co., Inc., Worthington, OH).
An apparatus useful in ASTM D 2172-81 testings is the Soiltest Vacuum Extractor Model AP-520 (Soiltest, Inc., Evanston, Ill.).